Primates are
remarkably recent animals. Most animal species flourished and became extinct long
before the first monkeys and their prosimian
ancestors evolved. While the earth is about 4.54 billion years old and the first
life dates to at least 3.5 billion years ago, the first primates did not
appear until
around 50-55
million years ago. That was10-15 million years after the dinosaurs had become extinct.

65,000,000 years ago

present

Transitional primate-like creatures
were evolving by
the end of the Mesozoic Era (ca. 65.5 million years ago). At that time, the
world was very different from today. The continents were in other locations and they
had somewhat different shapes. North America was still connected to Europe but not
to South America. India was not yet part of Asia but heading towards it
at a surprisingly rapid rate of nearly 8 inches (20 cm.) per year.
Australia was close to Antarctica. Most land masses had warm tropical or subtropical
climates.

The flora and fauna at the end of the Mesozoic Era would have seemed
alien since most of the plants and animals that are familiar to us had not yet
evolved. Large reptiles were beginning to be replaced by mammals as the dominant
large land animals. Among the mammals, there were a few archaic egg-layers
(monotremes) like the ancestors of the platypus and
echidna. There were larger numbers of pouched opossum-like mammals (marsupials). The few placental mammals that
existed at that time mainly consisted of the Insectivore ancestors of primates.
Most of the mammal species were small, ranging from about the size of a mouse
up to a medium size domesticated dog. The large
grass-eating placental mammals, such as cattle and wildebeest, were
absent as were the vast grasslands that would later develop. Rodents and
seed-eating birds were also absent. The great proliferation of flowering
plants had not taken place yet. However, forests of broad-leafed
trees were developing over much of the earth.

Primate-like Mammals

The first primate-like
mammals, or proto-primates,
evolved in the early Paleocene Epoch
(65.5-55.8 million years ago) at the beginning of the Cenozoic Era.
They were roughly similar to squirrels and tree shrews in size and
appearance. The existing, very fragmentary fossil evidence (from Asia,
Europe, North Africa, and especially Western North America) suggests that they were adapted to an arboreal
way of life in warm, moist climates. They probably
were equipped with relatively good eyesight as well as hands and feet adapted
for climbing in trees. These primate-like mammals (Plesiadapiformes) will remain rather shadowy
creatures for us until more fossil data become available.

The primate-like mammals
do not seem to have played an important role in the general transformation of terrestrial
animal life immediately following the massive global extinctions of plants and animals
that occurred about 65,500,000 years ago. The most dramatic changes were
brought about by the emergence of grazing and browsing mammals with tough hoofs,
grinding teeth, and digestive tracts specialized for the processing of grass, leaves, and
other fibrous plant materials. The evolution of these herbivorous mammals
provided the opportunity for the evolution of the carnivorous
mammals specialized to eat them. These new hunters
and scavengers included the evolutionary lines that would later produce the dogs, cats, and bears
of our time. Adaptive radiation was resulting in the
rapid evolution of new species to fill expanding ecological niches, or food getting
opportunities. Most of these new animals were placental mammals. With the
exception of bats, none of them reached Australia and New Guinea. This explains why
they did not exist there until people brought them in recent times. South America
had also drifted away from Africa and was not connected to North America after 80,000,000
years ago. However, around 20,000,000 years ago, South America reconnected with
North America and placental mammals streamed in for the first time, resulting in the
extinction of most of the existing marsupials there.

Early
Prosimians

The beginning
of the Eocene Epoch (55.8-33.9 million years ago) coincides with the
emergence of early forms of most of the placental mammal orders that are
present today. In addition, placental mammals with larger bodies and
bigger brains began to appear in the fossil record at this time. Paul Falkowski has suggested that this is due to the fact that the amount of
oxygen in the earth's atmosphere more or less doubled around 50 million
years ago. Larger mammals have relatively fewer capillaries for the
distribution of oxygen to the cells of their bodies. Subsequently,
they must breathe air that is more oxygenated. Brains have especially
high oxygen requirements. In addition, pregnant placental mammals must
transmit a substantial portion of the oxygen in their blood to their
fetuses. Coinciding with the increase in atmospheric oxygen at the
beginning of the Eocene Epoch was a relatively abrupt global warming of
9-16° F. (5-9° C.) lasting at least 200,000 years. This also would
have been a major factor in the rapid evolution of animals and plants at the
time. Overall, climates were significantly warmer during the Eocene
than now. There were crocodiles in the Arctic, pine forests in the
Antarctic, and palm trees in Wyoming. There was no polar ice. As
a consequence, sea levels were close to 330 feet (100 m.) higher than today.

The first
true primates evolved by 55 million years ago or a bit earlier, near the beginning of the
Eocene Epoch. Their fossils have been found in North America, Europe,
and Asia. They looked different from the primates today. They were
still somewhat squirrel-like in size and appearance, but apparently they had
grasping hands and feet that were increasingly more efficient in
manipulating objects and climbing trees. The position of their eyes
indicates that they were developing more effective stereoscopic vision as
well.

Smilodectes
(lemur-like family Adapidaefrom the Eocene Epoch)

Among the new primate species were many that
somewhat resemble modern
prosimians such as lemurs, lorises, and
possibly tarsiers. The Eocene was the epoch of maximum prosimian
adaptive radiation. There were at least 60 genera of them that were mostly in two families--the
Adapidae (similar to lemurs and lorises) and the
Omomyidae
(possibly like galagos and tarsiers). This
is nearly four times greater prosimian diversity than today. Eocene prosimians
also were much more widely distributed around the world than now. They lived in
North America, Europe, Africa, and Asia. It was during this epoch that they reached
the island of Madagascar, where they flourished up to modern times. The great diversity of Eocene prosimians was probably a
consequence of the fact that they did not have competition from monkeys and apes
since
these latter more advanced primates had not yet evolved.

47 million year old
Adapidae fossil
from Germany
(much of the soft
tissue, including
fur, is fossilized)

Major evolutionary changes
were beginning in some of the Eocene prosimians that foreshadow species yet to come.
Their brains and eyes were becoming larger, while their snouts were getting
smaller. At the base of a skull, there is a hole through which the spinal cord
passes. This opening is the foramen magnum (literally the "large hole or opening" in
Latin). The position of the foramen magnum is a strong indicator of the
angle of the spinal column to the head and subsequently whether the body is habitually
horizontal (like a horse) or vertical (like a monkey). During the Eocene, the foramen
magnum in some primate species was beginning to move from the back of the skull
towards the center. This suggests that they were beginning to hold their bodies erect
while hopping and sitting, like modern lemurs, galagos, and tarsiers.

Eocene Era primate
(on left) and modern
human skulls

By the end of the Eocene Epoch, many of the prosimian
species had become extinct. This may be connected with cooler
temperatures and the appearance of
the first monkeys during the transition to the next geologic epoch, the Oligocene (about 34 million years ago).

Early
Monkeys and Apes

Hornless rhinoceros (Indricotherium):
The largest known mammal from the
Oligocene Epoch

The body size of mammals in many species
lines progressively increased after the end of the age of dinosaurs as they
took advantage of the vast expanses of land and plant food made available by
the extinction of the giant reptiles. The biggest land mammals ever to
live evolved by around 39-40 million years ago (near the end of the Eocene
Epoch) and flourished during the subsequent Oligocene Epoch (33.9-23
million years ago). The largest of them was a hornless rhinoceros (Indricotherium
transouralicum) living in Eurasia that weighed 16.5 tons (15,000 kg.)
and stood 18 feet (5.5 m.) at the shoulders. By comparison, the
biggest African elephants today weigh 6.7 tons (6,046 kg.) and stand 13 feet
(4 m.) at the shoulders.

Unfortunately, the Oligocene Epoch was largely a gap in the primate
fossil record in most parts of the world. This is especially true for prosimian
fossils. Most of what we know about them came from the Fayum deposits in Western Egypt. While this area is a desert
today, 36-31 million years ago it was a tropical
rainforest on the edge of a large lake or sea. Other Oligocene deposits containing some fossil primate bones have been
found in North and West Africa, the Southern Arabian Peninsula, China, Southeast Asia, as
well as North and South America.

Old world monkey of the
Oligocene Epoch
(Aegyptopithecus zeuxis)

Monkeys evolved during the early Oligocene or
possibly near the end of the Eocene. Their ancestors were most likely
prosimians. These monkeys were the first species of our
infraorder--the Anthropoidea.
Several genera of early monkeys have been identified. Apidium and
Aegyptopithecus
are the most well known.
The former was about the size of a fat squirrel (2-3 pounds or .9-1.4 kg.), while the latter was the size of a
small dog (13-20 pounds or 5.9-9.1 kg.). Compared to the prosimians,
they had fewer teeth, less fox-like snouts, larger brains, and increasingly
more forward-looking eyes. These and other anatomical features suggest
that the early monkeys were becoming mostly diurnal
fruit and seed eating forest tree-dwellers.

New World monkeys appeared for the first time about 30
million years ago. It is generally thought that they began as isolated
groups of Old World monkeys that somehow drifted to South America either
from North America or Africa on large clumps of vegetation and soil.
The evidence suggests that Africa is the most likely continent of origin.
Such "floating islands" produced as a result of powerful storms tearing at
the land still occur in tropical regions of the world today. It is
likely that other kinds of small animals were transported to South America
in this way as well.

Due to the comparative
scarcity of Oligocene Epoch prosimians in the fossil record, it is generally believed that the monkeys out-competed and
replaced them in most environments at that time. Supporting this hypothesis is the fact that
modern prosimians either live in locations where monkeys and apes are absent or they are
normally active only at nighttime when most of the larger, more intelligent primates are
sleeping.

Great Rift Valley system of
East Africa
(shown in brown)
developed as tectonic plates
pulled apart beginning during
the Oligocene Epoch

Indian landmass is
continuously moving
north, pushing up the
Himalayas and the
Tibetan Plateau as a consequence

The Oligocene
was an epoch of major geological change with resulting regional climate shifts
that likely affected the direction of evolution and altered fossil
preservation conditions. By the beginning of the Oligocene, North America and Europe drifted apart and became
distinct continents. The Great Rift Valley
system of East Africa also was formed during the Oligocene along a 1200 mile
long volcanically active fault zone between tectonic
plates that are moving away from each other. This created an easy
north-south regional migration route for animals. Around 120 million years ago,
the tectonic plate that forms the Indian subcontinent began to rapidly drift
north across the Indian Ocean from Antarctica. By 50.5 million years
ago, India began crashing into Asia at a rate of 10-12 inches (25-30 cm.) a
year and continues to do so today. This has progressively forced up
the Himalayan chain of mountains and the high Tibetan Plateau beyond.
During the Oligocene, the continuing growth of this immense barrier altered
continental weather patterns significantly by redirecting the summer
monsoonal rains to the east. This created a vast arid rain shadow region in
Central Asia and very likely triggered global climate changes. The cooling
and drying trend with associated expansion of grasslands that had begun in the late Eocene Epoch accelerated, especially in the
northern hemisphere. A result was the general disappearance of primates from
these northern areas. However,
climates in most regions were still warmer than today.

By 16-14 million years ago, in the middle of the
Miocene Epoch
(23-5.3 million years ago),
the ongoing movement of tectonic plates in the Great Rift Valley system created new
volcanic mountain chains in east Central Africa.
These in turn altered local weather patterns. Some areas became wetter
while others more arid due to local rain shadows. In addition, the
progressive global cooling trend continued. Growing polar
ice caps reduced the amount of water in the oceans, causing sea levels to
drop. This exposed previously submerged coastal lands.
As a result of this and continental drift, a land connection was
reestablished between Africa and Eurasia
along the eastern Mediterranean Sea coast
that provided a migration route for primates and other animals between these
continents. Much of the East African and South Asian tropical forests
began to be replaced by sparse woodlands and dry grasslands because of the
climate changes. As a result, there were new selective pressures affecting
primate evolution.

Miocene Epoch monkey-ape
transitional genus
(Proconsul)

Proconsulskull

Primate
fossils are common from the Miocene. However, not all primates are
equally represented in the fossil record. Apes
apparently evolved from monkeys early in this epoch. Fossil monkeys and
prosimians are comparatively rare from most of the Miocene, but apes are common.
It appears that
apes at that time occupied some ecological niches that would later be filled by monkeys.
One of the earliest of the monkey to ape transitional primates was
Proconsul.
It lived in African forests 21-14 million years ago.

Among the numerous Miocene primate species
were the ancestors of all modern apes and humans. By 14
million years ago, the group
of apes that included our ancestors was apparently in the process of adapting to
life on the edges of the expanding savannas in
Southern Europe. They were very likely members of the genus
Dryopithecus,
which were generally similar in appearance to modern African apes.
These apes evolved mostly during a relatively short global heat wave that began around 15
million years ago. This caused enough polar ice to melt so that sea
levels once again rose 80-130 feet.

Toward the end of the Miocene, less hospitable cooler conditions in the
northern hemisphere once again caused many primate species to become extinct while some
survived by migrating south into Africa and South Asia where it remained
relatively warm. About 8-9 million
years ago, the descendants of the dryopithecines
in Africa diverged into two lines--one that led to gorillas and another
to humans, chimpanzees, and bonobos. Around 7 million years ago, a
further
divergence occurred which separated the ancestors of modern chimpanzees and
bonobos from the early hominins (human-like primates) that were our direct ancestors.

Summary

Primates
are relative newcomers on our planet. The earliest ones are found in the
fossil record dating to 50-55 million years ago. These first prosimians thrived during the Eocene
Epoch. There were no monkeys or apes for them to compete with yet. By the
time of the transition to the Oligocene Epoch, monkeys had begun to evolve from prosimians and became the dominant
primates. Many of the prosimian species became extinct probably as a
consequence. By the early Miocene Epoch, apes had evolved from monkeys and displaced them
from many environments. In the late Miocene, the evolutionary line leading to hominins finally became distinct. This hominin line included our
direct ancestors.